Here's a drawing of my own:
But I can't figure out how I would achieve this effect short of carving it with hand-tools. I could do an angled cove-cut with a table saw but I would need a blade about three feet in diameter. I could use a router maybe, but I'd need to make a very intricate jig and that would be as hard as the initial challenge. The only other thing I can think of is maybe using a band saw. I'd have to stand the board up vertically, and then at an angle and cut a curve. As if the band saw blade was mimicking a giant circular blade that just kissed the top of the board at a slight angle. Problem: I dont' have a band saw. Sounds dangerous anyway... so I'm still thinking. What if the chair back had no indentation at all? Less comfy, and less sleek I think. Luckily this can be one of the very last pieces I finish.